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Hozv Avoided.                               71

Hollow cylinders of all kinds are better moulded by inserting
an iron tube in the core, through which cold water is allowed to
circulate, and this can be so regulated as to produce a tensile
strain on the outside metal if needed, or, what is better, no strain
at all.

The pulley previously mentioned can be improved by curving
the arms, as in Fig. 82, thus giving them sufficient elasticity to take
the strain off the rim ; and such an example as the girder must be
left to the moulder's ingenuity, the thicker portions being first
uncovered, so as to cool quickly.

Of course it must be understood that a casting is never broken
directly by compressive stresses, but only by tensile stresses
induced by them in some other portion of the casting. Thus the
pulley will break at the arms, in which a tension is induced,
rather than at the compressed rim, although the latter may be
thin. This observation is true for all cases, (See$. 1012.) '

(For Plaster Patterns see Appendix EL, p. 781.)